The invention relates to a method for processing wood chips. The invention also relates to an apparatus for implementing the aforementioned method.
When manufacturing cellulose and paper pulp, the ligno-cellulose-containing wood chips used as raw material are cooked in an alkaline solution in order to separate the fibres and lignin contained in them from each other. The manufacture comprises several stages, both before and after the cooking. Before cooking, the wood chips are introduced to a gas removal stage, where gases are removed from both inside the wood chips and the spaces between them by directing hot steam to the wood chips. After the gas removal stage the wood chips are directed to the impregnation stage, where cooking chemicals are impregnated to the wood chips before the wood chips are directed to the cooking stage. Cooking takes place in an elevated temperature and it takes approximately 30 to 240 minutes, depending on the raw material and the cooking temperature. After cooking the lignin is separated from the lignin-fibre mixture by washing it in one or more washing phases. The remaining fibrous pulp is led for further processing, such as, for example, bleaching. The bleaching stage is also composed of several different phases, where the pulp is processed by means of bleaching chemical and scrubbed.
Nowadays pulp mills tend to use in pulp bleaching more bleaching methods that are based on the use of oxygenous chemicals, such as oxygen, peroxide and ozone bleaching. These oxygen-based bleaching methods are both ecological and economical and their implementation is beneficial in controlling the chemical recovery cycle of the mill, especially a so-called closed chemical recovery cycle.
The wood chips directed to the pulp digester naturally comprise organic compounds formed by metal ions and inorganic salts, for example, silicates, i.e. sand. Most common are compounds and salts formed by ions of sodium, potassium, calcium, magnesium, barium, iron, copper and manganese. Under the effect of the alkaline conditions in the digester a part of the metallic cations attached to anionic groups of wood dissolve in the cooking liquor by wood reactions and dissolution, or they react directly with the cooking liquor. A part of the metal compounds in the wood chips are very poorly soluble in alkaline conditions, and move on in the pulp manufacturing process as organic compounds combined to fibres or organic or inorganic compounds precipitated to the fibres. Poorly soluble metal compounds formed in the cooking liquor, for example calcium carbonate, tend to precipitate on the heat surfaces of the heat exchangers at the digester house and the evaporation plant because their solubility product decreases when temperature rises. On the other hand, potassium, which remains soluble in alkaline environment, causes problems in the black liquor combustion after cooking, which black liquor combustion is a part of the cooking liquor regeneration process. Potassium causes problems especially in the superheater part of the recovery boiler, by forming potassium chloride, which adheres to the steam tubes and is corrosive in high temperatures. Because of this the steam pressure and temperature of the recovery boiler must be lowered when the raw material of the pulp is chloridic wood chips. As a result of this the electricity production of the recovery boiler decreases and thus causes economic losses. Metals combined to the pulp, especially transition elements, cause problems in the bleaching performed later by means of oxygenic chemicals of the pulp. Metal ions for example catalyze the decomposition of oxygenic chemicals. Consequently, in order to reach the desired bleaching result, the chemical must be used in excess in the bleaching. This increases the chemical costs of bleaching. In addition, the metal ions catalyze side reactions that cut the cellulose fibres and thus cause pulp loss and deteriorated quality both in cooking and bleaching. They also cause the coloring of bleached pulp, mostly yellowing during storage. Further, metal ions, especially calcium ions together with the chemicals in the pulp suspension, form precipitations on the walls of the pulp production and finishing apparatuses, which precipitations cause clogs in the process apparatuses, as well as spotting of the finished pulp and other fouling.
Therefore, nowadays the aim is to remove metal ions from pulp in various ways. It is very common to remove metal ions from pulp coming from cooking by processing the pulp before the bleaching stage in a special metal removal stage, i.e. chelating stage (Q-stage). In this stage the pulp is washed with an acidic solution, to which a compound that is able to form complexes, i.e. chelates with the metal ions in the pulp has been added. Suitable chelating agents are, for example, ethylene diamine tetra acetic acid (EDTA) and diethylene triamine penta acetic acid (DTPA). These reagents are water soluble, easily soluble and commercially available. Another alternative is to wash the pulp after the acidic bleaching stage, in which case the metal ions remain in the washing water.
It has also been suggested that metal ions are removed from wood chips before they are directed to cooking. A method of this type is disclosed in patent application FI 20021152 (the corresponding US application 2003/0000661), where after the steaming of the wood chips, which takes place before cooking, the wood chips are treated with warm water solution. The water solution used includes water, condensate of the wood chip steamer or other condensate from the mill, rainwater or wastewater from the bleacher. As can be learned from the publication, this method is applicable only in removing potassium ion-containing compounds. The other metal ions in the wood chips do not react to this treatment and therefore remain in the wood chips.
Another method is disclosed in publication U.S. Pat. No. 6,413,367, wherein metal ions in the wood chips are removed by chelating chemicals used in producing pulp by the batch cooking method. According to the method during the packing of the digester is packed with wood chips, DTPA-containing water solution is fed to the steam used in packing the wood chips. DTPA forms chelates with the metal ions in the wood chips and the formed chelates are removed from the digester with the black liquor removed in the black liquor processing stage following the chelating stage. The solution according to the publication does remove the problems caused by metal ions in connection with pulp cooking and processing, but it transfers them to the processing of black liquor. That is, the chelates formed by the DTPA decompose in high temperatures. The metal chelates removed from the digester with black liquor end up at the evaporation plant, where they decompose and cause problems there by precipitating on the heat delivery surfaces.